This invention relates to a method of forming helical splines with stoppers on a rotary shaft, and rolling tools for practicing the method.
In an example of a starter motor, as disclosed by Japanese Utility Patent Application (OPI) No. 53727/1980 (the term "OPI" as used herein means an "unexamined application") its output shaft has helical splines with stoppers, and it is spline-coupled to the sleeve of the clutch outer of an overrunning clutch while being prevented from moving axially by the stoppers.
A method of forming the output shaft will be described with reference to FIG. 5. First, as shown in the part (A) of FIG. 5, a round rod of steel is cold-forged and machined into a shaft 1 having a flange 1a, a relief 1b, a large-diameter portion 1c, and a small-diameter portion 1d. Thereafter, as shown in the part (B) of FIG. 5, the large-diameter portion 1c is rolled being held between a pair of rolling tools 2 which are set on the shaft 1 in such a manner that they are turned 180.degree. from each other.
Each of the rolling tools 2 is as shown in the part (A) of FIG. 6. In the part (a) of FIG. 6, 3 designates a helical teeth forming die having a plurality of inclined teeth 3a; and 4, an aligning teeth die having a plurality of aligning teeth 4a, which are aligned with every other inclined tooth of the helical teeth forming die 3. The helical teeth forming die 3 and the aligning teeth die 4 are joined to each other as shown in the part (B) of FIG. 6, thus forming the rolling tool.
The part (C) of FIG. 5 shows a rotary shaft 5 which is formed by rolling the large-diameter portion of the shaft 1 as shown in the part (B) of FIG. 5. That is, the large-diameter portion 1c of the rotary shaft 5 has helical splines 6 having a plurality of helical grooves 6a (for instance ten helical grooves), communication grooves 7a communicated with every other helical groove 6a, and stoppers 7 between the communication grooves 7a.
Manufacturing the rotary shaft 5 suffers from a difficulty in that small burrs 8 are liable to be formed between the helical grooves 6a and the communication grooves 7a communicated with the latter 6a. That is, each rolling tool 2 is formed by combining the helical teeth forming die 3 and the aligning teeth die 4 with each other. In this tool forming operation, there is formed a small gap between the dies 3 and 4 depending on the machining accuracy of them. The small gap thus formed results in formation of the burrs 8 between the helical grooves 6a and the communication grooves 7a. The burrs 8 thus formed give rise to difficulties as follows: When an element to be driven by the rotary shaft is to be spline-coupled to the latter, the burrs 8 obstruct the movement of the element along the rotary shaft. The burrs 8 may come off to enter the spaces between the splines, thereby to obstruct the sliding of the element. In order to eliminate the above-described difficulties, heretofore the burrs are removed by hand finishing.
In order to eliminate the formation of the burrs 8 at the helical grooves 6, another method of forming helical splines with stoppers has been proposed in the art. The method is as shown in FIG. 7. That is, as shown in the part (A) of FIG. 7, a round rod of steel is cold-forged and machined into a shaft 1 having a flange 1a, a relief 1b, a large-diameter portion 1e, an annular groove 1f, a stopper 1g, and a small-diameter portion 1d. In the shaft 1 thus formed, the outside diameter of the annular groove 1f is smaller than the diameter of helical grooves 6a and communication grooves 7a which are formed on the shaft 1 by rolling it later. The shaft 1 is processed as follows: Similarly as in the case of the part (B) of FIG. 5, the shaft 1 is rolled under pressure with the large-diameter portion 1e and the stopper 1g held between a pair of rolling tools 2. The shaft 1 thus rolled has helical splines 6 having a plurality of helical grooves 6a, communication grooves 7a aligned with every other helical groove 6a, and stoppers 7 between the communication grooves 7a. That is, an aimed shaft 9 has been formed. In the rolling of the shaft 1, no burrs are formed, because when the helical teeth forming die 3 and the aligning teeth die 4 are set on the shaft 1, the junction of those dies is located over the annular groove 1f.
The conventional method described with reference to FIG. 5, as was described above, is disadvantageous in that the burrs 8 are formed between the spline grooves 6a and the communication grooves 7a, and therefore it is necessary to remove the burrs 8; that is, the method is low in productivity.
The conventional method described with reference to FIG. 7 which has solved the problem accompanying the method shown in FIG. 5 provides another problem to be solved. In the method, the shaft 1 has the annular groove 1f; that is, the number of manufacturing steps is increased as much. Because of the provision of the annular groove 1f, the spline-coupling length of the helical splines 6 with the clutch outer of the over-running clutch is decreased as much, and the bearing stress is higher. This problem may be solved by increasing the length of the helical splines as much as the width of the annular groove 1f. However, increasing the length of the helical splines in this manner gives rise to another problem in that the rotary shaft 9 is increased in length as much.